The present invention relates to liquid fueled rocket engines, and more specifically, to injectors for liquid fueled rocket engines.
In the operation of rocket engines which employ liquid propellants especially liquid fuel and oxidizer materials, the propellants are injected into a combustion chamber through the face of an injector. The orifices through which the propellants pass are quite small in diameter, often under ten one-thousandths of an inch. In addition, the orifices are often formed in pairs with the individual orifices being angled so that the injected liquid streams meet at a point, called the impingement point, in the combustion chamber.
Forming the injector face with such small orifices and with all orifices having the correct orientations presents difficult fabrication problems and is expensive to accomplish. In many cases the injector body must be assembled from a number of subassemblies, and when this is the case it is important that the joints do not offer leakage paths through which an explosive mixture of fuel and oxidizer could inadvertently be formed. Since an injector with a misdrilled or damaged injection orifice is often not acceptable, it is desirable that the orifices be drilled in smaller subassemblies which are subsequently joined to form the entire injector. In this way a single damaged orifice will only require that one subassembly be discarded, and not the entire injector. To perform as intended, pairs of streams which are intended to impinge upon each other must have their centerlines located with an accuracy of a few thousandths of an inch. It is very difficult to machine, assemble and weld or braze subassemblies so that the holes in one subassembly will accurately impinge with the streams from the corresponding holes in a different subassembly. Hence, it is desirable that all pairs of holes which are intended to impinge should be drilled in the same subassembly. When it is desired to have a fuel stream impinge upon an oxidizer stream, it is especially difficult to simultaneously fulfill all of the above requirements.
One of the more common methods which has been used to fabricate injectors is to cast an injector blank with the manifolds cast in place, and then to drill the injection orifices through the injector face to intersect the manifold passages. However, with this approach, if a bad hole is drilled the entire injector is often rendered useless.
Another common method of fabrication is to machine circumferential flow passages into the back side of the injector blank, with the injection orifices drilled through the injector face to intersect the circumferential passages. After the drilling is complete, the circumferential passages are sealed off by welding or brazing rings into place. Again, with this approach, one poorly drilled hole may render the entire injector useless.
Yet another common method of construction is to machine manifolds into the injector face in the form of circumferential slots. These slots are later sealed off with rings which have previously been drilled with the injection orifices, and which are welded or brazed in place. However, with this approach corresponding pairs of holes are not drilled in the same subassembly and a slight displacement of the rings causes poor atomization and resultant poor performance.
When a rocket engine is to be used to repetitively fire very short duration burns (i.e. in pulse-mode operation), then the volumes of the flow passages between the valves and the injector face (i.e. the dribble volumes) are very important. Propellant is left in these passages at the conclusion of each pulse and is largely wasted by evaporation into a vacuum environment. Minimization of the volumes of these passages is imperative for efficient pulse-mode operation; however, to date no injector has been designed which fulfills the above-mentioned requirements while minimizing the dribble volume.